The police were focusing their search on northern France, where the suspects were reported to have stolen food from a gasoline station. One of the cars they used in their getaway from Paris on Wednesday was found abandoned in the area.

Television channels carried live coverage of the search for the brothers, Saïd and Chérif Kouachi, 34 and 32. A third suspect, Hamyd Mourad, 18, turned himself in early Thursday at a police station in Charleville-Mézières, about 145 miles northeast of Paris.

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Journalists at Agence France-Presse in Paris held signs Thursday reading “Je suis Charlie” (I am Charlie) during a moment of silence for the victims of the attack at the newspaper Charlie Hebdo.CreditFrancois Xavier Marit/Agence France-Presse — Getty Images

The sighting of the brothers and the discovery of the car in the town of Villers-Cotterêts, in Picardy, captivated a nation that seemed to draw together on Thursday, at least for a moment of silence at noon on a rare official day of national mourning, to defend French values like freedom of the press and religious tolerance.

French police officers were also guarding railway and subway stations, department stores and other potential targets like news media offices on Thursday. Officials in Britain said they were stepping up screening at ferry ports in case the two men tried to head that way.

In London, the head of Britain’s MI5 Security Service, roughly equivalent to the F.B.I., warned on Thursday that Qaeda militants in Syria were plotting attacks to inflict mass casualties in the West, possibly against transport systems or “iconic targets.”

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French police questioned a motorist in Longpont, France, on Thursday.

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Andrew Parker, in a speech prepared before the Paris attack but delivered afterward, called an attack on Britain highly likely. About the Paris attack, he said, “it is too early for us to come to judgments about the precise details or origin of the attack, but it is a terrible reminder of the intentions of those who wish us harm.”

In Washington, President Obama visited the French Embassy on Thursday evening, where he signed a book of condolences and paused for a minute of silence alongside Ambassador Gerard Araud, according to the White House pool report.

The moment of silence in France was widely respected, but the national mood encompassed fear, anger, unity and, ultimately, defiance. Isolated events helped fan anxiety. On Thursday morning, a police officer was killed and a city employee was wounded by gunfire near a subway station just south of Paris.

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Chérif Kouachi, one of the suspects in the Charlie Hebdo shootings in Paris, appeared in a 2005 investigative documentary about jihadism that aired on French television.CreditCreditPièces à Conviction, France3

The police said the shooting appeared unrelated to the attack on the newspaper or the subsequent manhunt, and they announced two arrests. There was also an explosion at a kebab shop in eastern France, with no casualties reported, and two mosques were fired at, prosecutors said.

The government moved quickly to try to capture the fugitive brothers and reassure the nation. The police said they had detained several people for questioning in the case and had arrested some of them. They also announced that at least five planned terrorist attacks had been thwarted in France over the last 18 months. The alert level in northern France and in Paris, the capital, was raised to the highest level.

The police had their first big break in the case when they discovered that one of the brothers, Saïd, had left his identity card in the first car used by the gunmen, which was found abandoned Wednesday evening after a crash. The police reportedly found Molotov cocktails and jihadist banners in the car as well.

The authorities seemed to catch another break when the men robbed the gas station. The manager of the station said they were wearing masks and waving Kalashnikov assault rifles, like the ones used in the attack on the newspaper office.

Xavier Castaing, a spokesman for the Paris police, said that two men fitting the description of the suspects were spotted in Villers-Cotterêts driving a gray Renault Clio, the same kind of car that the two fleeing suspects had hijacked late Wednesday.

In the evening, a police search appeared to be scouring a stretch of forest just to the west of Longpont, a small town on the N2 highway between Villers-Cotterêts and Soissons. Officers blocked off a road leading into Longpont, and at least 50 journalists from as far away as Japan and Russia huddled in the cold and damp. At noon, in a sign of mourning for the worst terrorist attack in France since the Algerian war, bells rang, schools stopped classes, and corporate meetings were cut short. At mosques, people bowed their heads. Some electronic road signs displayed the words, “Je suis Charlie” — I am Charlie.

At Notre Dame Cathedral, pedestrians wept as hundreds stood silent on a gray and rainy day to pay tribute to the victims. Dozens placed flowers in front of Charlie Hebdo’s headquarters. Through the vigils and tributes, some people held pencils, a symbol of support for freedom of the press. There was a palpable sense of determination that France and its vaunted Republican values of free speech and freedom of expression would not be subverted by religious extremism.

Charlie Hebdo announced that despite the loss of so many of its people — including cartoonists who have been well known for a generation — it would publish as scheduled next Wednesday, and would print 1 million copies, rather than the usual 60,000.

Many Muslim clerics and spokesmen added their voices to the sense of outrage, decrying the killings as antithetical to legitimate Islam.

Abdennour Bidar, a French Muslim and professor of philosophy, said on Arte television that the killers “do not deserve the name of Muslims.” In the name of Islam, he said, he would not allow Islam to be “instrumentalized, stolen by these people who say that they are avenging the Prophet.”

“It’s a disgrace, an infamy, a lie,” he said.

But in a sign of how the attack was already spilling over into French politics, Marine Le Pen, leader of the far-right National Front, called on Thursday for a national referendum on whether to reinstitute the death penalty. “The Islamists have declared war against France,” she told a French broadcaster.

Ms. Le Pen said that she had not been invited to join a “unity rally” on Sunday. “Things are clear from now on — the masks fall off,” she told Le Monde newspaper. “National unity is a pathetic political maneuver.”

Correction:Jan. 9, 2015

Because of an editing error, an earlier version of this article referred incorrectly to the shooting of a police officer and a street sweeper in a southern Paris suburb. Only the officer was killed; the street sweeper survived.

Reporting was contributed by Maïa de la Baume from Paris; David Jolly from Longpont, France; Ravi Somaiya from New York; and Elena Schneider from Washington.

A version of this article appears in print on , on Page A9 of the New York edition with the headline: As France Mourns, Police Hunt for Suspects in Newspaper Attack. Order Reprints | Today’s Paper | Subscribe